This invention relates to a two-step method for providing a polyolefin resin substrate with an adhesive layer comprising the steps of coating the surface of a polyolefin substrate with a hydroxy hydrocarbon polymer paint (primer) film containing a catalyst which promotes the reaction between the OH and the NCO radicals; and coating the paint film with a molten layer of a moisture setting hot melt urethane adhesive coating of urethane prepolymers containing NCO radicals which crosslink with the OH groups in the primer coating.
Generally a polyolefin resin such as polyethylene, polypropylene, ethylene propylene diene rubber (EPDM) and the like are low in adhesion, so that various methods have been used to provide adhesive properties to these resin substrates. For example, there is a method to improve adhesion by conducting electric and chemical oxidation treatment to the surface of a polyolefin resin substrate to approve the quality of the surface. There is also a method to improve adhesion by undercoating various primer components on the surface. Primers containing chlorinated polypropylene, isocyanate compound, an addition compound of isocyanate compound to polyhydroxy polyolefin (Japanese Patent Application No. 59-124937), and a reaction product of polybutadiene glycol having hydroxy groups at the end and polyisocyanate are known in the prior art. ( Japanese Patent Application No. 62-95326).
However, in the case of using a thermoplastic resin such as chlorinated polypropylene among said primers, sufficient adhesiveness cannot be ensured, and heat resistibility reaches the softening point of the resin (for example, polypropylene chloride: 60.degree. C. to about 70.degree. C.), at a temperature not exceeding 100.degree. C. An addition compound of polyhydroxy polyolefin and isocyanate compound can expect high heat resistibility, but an improvement of the effect of an adhesiveness to a polyolefin resin is scarcely seen. Its coated liquid reacts with moisture in the air to change the prefer components. Also, the paint film after coating reacts with moisture in the air. The disadvantages are that the adhesiveness is not constant according to a timing of coating of adhesive to be used next to make a control for the adhering process intricate. As for said addition compound, there are disadvantages that if the isocyanate component is used too much, the adhesiveness and thermal resistance are lower than in a case that polyhydroxy polyolefin alone is used. A reaction product of polybutadiene glycol having a hydroxy group at the end, and polyisocyanate is not sufficient in all points such as primary adhesion, thermal resistance and so on.
On the other hand, a method which incorporates the primers mentioned above, and the crosslinked moisture setting urethane prepolymers which is a tacky liquid at room temperature has been developed in order to improve the heat-resistance of the whole adhesive system. However, according to this method, said adhesive does not always provide tackiness at an early stage. However the crosslinking of the NCO and OH groups provides tackiness, but does not provide an initial setting strength and initial bond strength. Therefore, in order to compensate for the lack of the initial setting strength and initial bond strength, pressing the polyolefin resin and adherent with a pressing machine is conducted. However, in these cases, apparatus and hands and space for pressing are required so that the apparatus or the process for adhering becomes a large scale operation. Even if other reaction type adhesives are used or substituted for said adhesives, and since they are usually two-part mixing adhesives (main agent/curing agent), they require hands for mixing, and a mistake in the mixing ratio can occur which could result in a material loss. Moreover, since these two-part mixing type adhesives have slow reaction speed at room temperature, they require said presser and the like, and it is very expensive to press by hand.
Accordingly, an object of this invention is to provide a method for providing a polyolefin resin substrate with an adhesive coating having high adhesion and thermal resistance, wherein said operation is quite easy. A superior adhesiveness can be obtained at an initial stage of the adhesion, and at the same time the heat resistance of the glue layer can be ensured, and the polyolefin resin can be coated with a material via the adhesive coating on its surface.